The basic premise of polymorphism, one might argue, can be likened to Plato’s theory of Forms. Plato believed that everything in the world was an imperfect representation of its true Form. A rock on the ground could be thought of as an inexact approximation of its true Form: “Rock”. Likewise, that chair in your room is merely mimicking the perfect, idealized form of a chair. So somewhere—perhaps not in physical form—there exists a pure, perfect, and authoritative version of a chair that all chairs here on Earth mimic, but never quite get 100% right.

It may sound a bit existential, but what Plato was really talking about (from a computer science point of view) was methods of categorization. Think about it. How hard would it be to get a computer to recognize that chair in your room as a chair and not a lamp? By the same token, it’s very easy for us to classify it as a chair. When we classify things on the spot like this, we’re comparing what we see with our own mental vision of what a chair’s “true Form” is.

What is its true Form? What makes a chair a chair, or a door a door? Well, a chair will typically have 3 or more legs, a place to rest your bum, and a back you can lean against. Doors are harder to pin down, but they are generally embedded in walls, and will open somehow to let you go through that wall.

For now, let’s take the example of a door and try to point this ship back towards the topic at hand: polymorphism. Suppose we wanted to build some code to mimic a door. What would it need to do? Ideally it would open. It should probably also be in a wall of some kind. Some basic height and width dimensions might be handy too. So, if I may, this might be your pseudo-code for a door:

door {
action: open();
property: wallType;
property: width;
property: height;
}

What we just pseudo-created is, arguably, the “perfect” Form of a door. Really, we just created a door that’s so generalized it’s pretty much useless. What we really need is an instance of this door, that we can use and go through:

supermarketDoor is-a door {
action: open(
action: slideOpen();
);
property: wallType = "glass";
property: width = "12 feet";
property: height = "8 feet";
}

And that’s it. We just polymorphed. Did you feel it? We can now use our supermarketDoor object. But, from a programming point of view, what allows us to use this door is not that we know how it works. Rather, we can use this door because we know it is a door, and as such has certain characteristics that we know about. We know this door, or any other door for that matter, will be a predicable variation of our original “true Form” of a door. It will open, it will be in a wall of some type, and it will have height and width dimensions. The fact that this door slides open isn’t important, we just want it to open when we tell it to.

So now we can write a quick bit of code that tells a given door to open. And since we know all doors mimic the Form of a door, we can rest assured that our code will always work—no matter what kind of door it’s presented with. This allows us to create a million different types of doors, and never have to touch our original code that opens a door again. That’s the power of polymorphism.